docs: def PartialDynamicSystem(self, ieq, variable):
          """
          returns dynamical system blocks associated to output variable
          """
          if ieq == 0:
              # U2-U1=signal
              if variable == self.physical_nodes[0].variable:
                  # U1 is output
                  # U1=U2-signal
                  return [WeightedSum([self.physical_nodes[1].variable, self.voltage_signal], variable, [1, -1])]
              elif variable == self.physical_nodes[1].variable:
                  # U2 is output
                  # U2=U1+signal
                  return [WeightedSum([self.physical_nodes[0].variable, self.voltage_signal], variable, [1, 1])]